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Abstract

The plant hormone gibberellin (GA) promotes several processes during Arabidopsis reproductive development, including the transition to flowering, floral organ growth and fertility. GA functions during stamen development to promote degradation of the tapetum cell layer through programmed cell death (PCD) and in post-anthesis pollen development. Bioactive GA is synthesised through a multi-step pathway, in which the last two biosynthetic steps are expressed as conserved multigene families. One of these, the GA 20-oxidases (GA20ox) consists of five paralogues in Arabidopsis, though physiological functions have only been ascribed to two (AtGA20ox1 and -2). Through a reverse genetics approach, this project demonstrates that AtGA20ox1, -2 and -3 account for almost all GA20ox activity in Arabidopsis, with very little evidence of any functions for AtGA20ox4 or -5. Unlike AtGA20ox1, -2, -3 and -4, AtGA20ox5 possesses only partial GA20ox activity, performing the first two out of three sequential catalytic conversions in vitro.

Partial functional redundancy occurs between AtGA20ox1, -2 and -3 across Arabidopsis development, although AtGA20ox1 and -2 dominate. Mapping of floral AtGA20ox expression through qPCR and the creation of transgenic GUS reporter lines found that the relationship between these three paralogues is complex, and not explicable through the simple hypothesis of co-expression in the same tissues. During anther development, the reported expression of AtGA20ox1, -2, -3 and -4 is mainly restricted to the tapetum cell layer, and loss of AtGA20ox1, -2 and -3 results in an anther developmental arrest in which the tapetum does not degrade.

This project demonstrates that stamen development is dependent on an optimum level of GA, with GA-deficiency restricting filament elongation to prevent pollination and GA-overdose negatively affecting anther development. DELLA repression of GA signalling is necessary for successful pollen development, with two of the five DELLA paralogues, RGA and GAI, critical to this process in the Columbia ecotype.